Switchgrass is a promising biofuel alternative to corn, but farmers, environmentalists
and biofuel developers, find deciding on the right time to harvest particularly thorny.

Generating biofuels from agricultural sources is nothing new; the United States has
been using corn to generate ethanol for decades. Until now, corn has been a passable
biofuel source, but it is an annual crop requiring yearly planting, costly fertilization
and irrigation, as well as a narrow harvest window dependent on grain maturity. The
benefits of using dedicated energy crops, crops grown with the sole purpose of being
used to create fuels, give plants like switchgrass a hefty leg up over corn.

Switchgrass, unlike corn, is a low-resource perennial plant. This not only saves on
replanting costs—switchgrass stands are expected to last for 20 years—but also allows
its root system to stay in the ground, reducing risk of erosion and run-off while
channeling fertilizing nitrogen back into the soil. Switchgrass serves as a wildlife
habitat for birds and predatory insects and is exceptional at sequestering carbon
into the soil, meaning it can pull carbon dioxide from the air to help combat greenhouse
gas emissions. And since switchgrass harvest isn’t dependent on grain maturity, farmers
have more options when it comes to timing.

Energy Crop Harvesting: When is Best?

The current viable harvest window for switchgrass extends from peak biomass, when
the grass is at its biggest before it begins to degrade from weathering, to the first
killing frost, when the plant shuts down completely for the winter. The longer the
plant stays in the field, the more nitrogen returns to the soil—a clear environmental
benefit—but this extra time reduces the amount of biomass harvested from the crop.
Future biofuel manufacturers may want to harvest multiple times per season to efficiently
use storage facilities, while farmers must consider weather and the harvest times
of their other crops. A conundrum.

Nailing down a definitive answer to the switchgrass harvest debate is something Rebecca Ong, assistant professor of chemical engineering, is exploring in her research. Working with researchers from the University of Tennessee,
the University of Wisconsin-Madison and Michigan State University, Ong monitored and
sampled several stands of Wisconsin switchgrass to better understand the costs and
benefits to harvesting at various points in the season. The work is part of the Great Lakes Bioenergy Research Center.

"A lot of times there seems to be this disconnect between our processing requirements
and our environmental requirements."Rebecca Ong

Microbes Shake Up the Data

Once harvested, switchgrass doesn’t magically become biofuel; the crop has to be broken
down and fermented to convert its natural sugars into ethanol. Harvest times affect
conversion efficiency and research until Ong's has demonstrated that, generally, earlier is better.

Comparing data from samples collected every two to three weeks from the switchgrass
fields against biofuel yield data from each site, Ong discovered something rather
unusual: As the crop aged past senescence, the point at which plants begin to go dormant
for the winter, ethanol yields from the harvested crop became more efficient rather
than less so.

Since switchgrass harvest isn’t dependent on grain maturity, farmers have more options
when it comes to timing. Credit: Great Lake Bioenergy Research Center

Ethanol is produced from energy crops like switchgrass in a two-step process: First,
using chemical and biological processes to break down the plants’ cell walls, accessing
and splitting the complex polymeric sugar chains into individual sugars; then applying
biological processes (usually microbe-aided fermentation) to convert the sugars into
ethanol.

The types of sugars present in each energy crop vary, and the microbes often need
some persuading to utilize certain sugars like xylose, one of those found in switchgrass.
Previous researchers at Purdue University have engineered yeast, the microbe involved
in switchgrass fermentation, to break down xylose, but the process can be fickle and
sensitive to inhibition.

“Something we’re trying to do [in our lab],” Ong says, “is break up that polymer,
break all the links so that we have those individual sugars and then convince the
microbes that they want to actually eat them.”

In the later samples from Ong’s study, the yeast didn’t need convincing. The xylose
had become consumable.

Ong believes this change is due to plant senescing, which is when plants begin to
go dormant and dismantle compounds in their stems and leaves for storage and reuse
the following growing season. When the switchgrass was harvested before senescence,
one or more of these compounds might have been interfering with the microbes’ ability
to digest the xylose. Giving the plants more time in the field may have made the switchgrass
sugar more appealing to the yeast. “Most of the time,” Ong says, “people say ‘harvest
early, get the highest conversions,’ but no one has taken it through fermentation
to see what the microbes do in the process.”

"It’s the first time that anyone has actually seen a negative impact on the process
side from harvesting early."Rebecca Ong

Sooner or Later, It All Goes to Seed

And while this might be good news for the environment and the biofuel manufacturing
process, this much later harvest window puts a strain on farmers who are limited by
weather, regardless of fascinating sugar behaviors.

"Previously, people had been saying ‘Well, you can harvest any time. Earlier is better
[for processing], but you can really harvest any time.’ And what we’re finding is
that this may not be true. So, you may have a more limited harvest window than we
were expecting."Rebecca Ong

The next step, according to Ong, is to figure out what compound or compounds are inhibiting
the xylose conversion earlier in the harvest season. Once identified, they can deploy
strategies to mitigate the effects, like engineering the biofuel-making microbes to
be more tolerant, giving farmers more room to harvest switchgrass during good weather.

Michigan Technological University is a public research university, home to more than
7,000 students from 54 countries. Founded in 1885, the University offers more than
120 undergraduate and graduate degree programs in science and technology, engineering,
forestry, business and economics, health professions, humanities, mathematics, and
social sciences. Our campus in Michigan’s Upper Peninsula overlooks the Keweenaw Waterway
and is just a few miles from Lake Superior.

About the Researcher

Rebecca Ong

Research Interests

About the Author

Paige Short

Paige is a student writing intern in University Marketing and Communications pursuing
a degree in Scientific and Technical Communication, minoring in both Media Production
and Psychology. She is also the Consultant and Resource coordinator for the Humanities
Digital Media Zone. In her spare time, Paige loves playing table-top games and video
games, cooking exciting new things, and spending time with her family.